【FLUTTER ANDROID STUDIO and IOS】Managing State

 import 'package:flutter/material.dart'

;import 'package:provider/provider.dart';

void main() {
  runApp(
    ChangeNotifierProvider<Counter>(
      child: MyApp(),
      create: (_) => Counter(),
    ),
  );
}

class Counter with ChangeNotifier {
  int _count = 0;

  void increment() {
    _count++;
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hello'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            MyText(),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () =>
            context.read<Counter>().increment(),
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class MyText extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text('${context
        .watch<Counter>()
        ._count}',
        style: Theme
            .of(context)
            .textTheme
            .headline4);
  }
}